A **spectrum** is a usually 2-dimensional plot, of a compound signal, depicting the components by another measure. Sometimes, it is used to refer to the compound signal itself, such as the "spectrum of visible light", a reference to those electromagnetic waves which are visible to the human eye.

The spectrogram is the result of calculating the frequency spectrum of windowed frames of the signal.

There are many specific meanings of spectrum:

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- The Spectrum was the sports arena in Philadelphia where the Philadelphia Flyers Hockey team and the Philadelphia 76ers Basketball team played untill the First Union Center was built; it is still located, used largely by the Philadelphia Phantoms, in the sports complex on Broad Street in Philadelphia.

- The frequency spectrum is the result of a frequency transform of a mathematical function into the frequency domain.
- The spectrum of an operator has to do with the invertibility of an operator in function spaces.
- The spectrum of a matrix is basically the spectrum of an operator where the matrix is considered as operator. Precisely, it is the set of the matrix's eigenvalues.
- The spectrum of a ring is the set of prime ideals of a ring.
- A (strange) construction, similar to the frequency spectrum, is the cepstrum of the quefrency.

- See: Timbre. Spectra is one of the determinants of the timbre or quality of a sound. It is the relative strength of pitches called harmonics and partials (collectively overtones) at various frequencies usually above the fundamental frequency, which is the actual note named (eg. an A).

- The electromagnetic spectrum is the power spectrum of an electromagnetic signal - see: electromagnetic spectrum, spectroscopy
- The optical spectrum is the electromagnetic spectrum of visual light
- The power spectrum is the distribution of the energy of a function in the frequency domain, which is actually the same as the magnitude of the frequency spectrum. See spectroscopy.

- The spectrum of activity of an antibiotic evaluates how wide a range of infections can be treated.

- There is a political spectrum which is said to go from left to right.

- The term spread spectrum is used for certain kinds of signal transmission.

**Spectrum**is the name of the defence force of Earth in the science fiction television series Captain Scarlet.- The
**ZX Spectrum**was also the name of a 1980s personal computer*see*ZX Spectrum.